High-precision barium isotope measurements yielded differences of up to 25 parts per million in the 137Ba/136Ba ratio and 60 parts per million in the 138Ba/136Ba ratio between chondrites and Earth. These differences probably arose from incomplete mixing of nucleosynthetic material in the solar nebula. Chondritic meteorites have a slight excess of supernova-derived material as compared to Earth, demonstrating that the solar nebula was not perfectly homogenized upon formation.
Measurable variations in
182
W/
183
W,
142
Nd/
144
Nd,
129
Xe/
130
Xe and
136
Xe
Pu
/
130
Xe in the Earth and meteorites provide a record of accretion and formation of the core, early crust and atmosphere. These variations are due to the decay of the now extinct nuclides
182
Hf,
146
Sm,
129
I and
244
Pu. The
l82
Hf–
182
W system is the best accretion and core-formation chronometer, which yields a mean time of Earth's formation of 10 Myr, and a total time scale of 30 Myr. New laser shock data at conditions comparable with those in the Earth's deep mantle subsequent to the giant Moon-forming impact suggest that metal–silicate equilibration was rapid enough for the Hf–W chronometer to reliably record this time scale. The coupled
146
Sm–
147
Sm chronometer is the best system for determining the initial silicate differentiation (magma ocean crystallization and proto-crust formation), which took place at
ca
4.47 Ga or perhaps even earlier. The presence of a large
129
Xe excess in the deep Earth is consistent with a very early atmosphere formation (as early as 30 Myr); however, the interpretation is complicated by the fact that most of the atmospheric Xe may be from a volatile-rich late veneer.
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